Carbonaceous soil amendments to biofortify crop plants with zinc

Carbonaceous soil amendments, comprising mixtures of biosolids and biochar, have been demonstrated to improve fertility while reducing nitrate leaching. We aimed to determine the efficacy of a biosolids/biochar soil amendment in biofortification of vegetables with Zn, an element that is deficient in one third of humanity. We grew beetroot (Beta vulgaris), spinach (Spinacia oleracea), radish (Raphanus sativus), broccoli (Brassica oleracea), carrot (Daucus carota), leek (Allium ampeloprsum), onion (Allium cepa), lettuce (Lactuca sativa), corn (Zea mays), tomato (Solanum lycopersicum), and courgette (also called zucchini - Cucurbita pepo) in an unamended soil (silt loam, pH 5.6), and soil amended (by volume) with 10% biosolids, 20% biochar, and 10% biosolids+20% biochar. The biosolids and biosolids+biochar treatments significantly increased the biomass and Zn concentration of most species, with a large interspecific variation. Beetroot showed the greatest increase, with dry weight Zn concentrations of up to 178 and 1200 mg kg(-1) in the bulbs and leaves respectively. Cadmium, Cu and Pb were below guideline levels in all samples, except the leaves of spinach and beetroot, which slightly exceeded the World Health Organisation's maximum permitted concentration of 0.1 mg Cd kg(-1) fresh weight. A mixture of biosolids and biochar is an effective means to biofortify crops with edible leaves as well as beetroot with Zn. Future research should investigate the efficacy of the system in other soil types and the role of biochar in the immobilisation/inactivation of organic contaminants and pathogens contained within the biosolids.